As known, the information associated with a data signal, having a bandwidth W located in a frequency band bounded by lower and upper limits f.sub.c and f.sub.c + W, can be completely obtained in baseband from samples of that signal extracted by means of an in-band sampling operation carried out at a suitable frequency f.sub.s satisfying the sampling theorem, i.e. f.sub.s .gtoreq. 2W.
Such an in-band sampling operation is necessary to realize a demodulator having a completely digital structure, yet it gives rise to a lot of problems which are not easily solved.
First, the sampling frequency, besides being compatible with the sampling theorem, must take into account the characteristics of the transmission system where the demodulator is to be inserted (more particularly, the band of the transmitted signal and the location thereof); it must satisfy at the same time the requirements relative to the signal demodulation, and particularly relative to the coherent demodulation and to the bit synchronization; moreover, it must be able to avoid aliasing phenomena, i.e. the phenomena of overlapping of repetitive spectra of the signal, which are characteristic of the sampling itself.
A second problem is due to the fact that in-band sampling on the one hand must be carried out in instants suitable to the recovery of the information (bit synchronization) and on the other hand must yield a true signal demodulation. Thus, an interaction may occur between possible demodulation-coherence errors and bit-synchronization errors, which could considerably degrade the performance of the system and must therefore be avoided as far as possible.
Moreover, the sampling must be carried out in such a way as not to suffer from possible frequency shifts of the band of the transmitted signal (frequency offset).
A further problem arises when the demodulation must be of coherent type. In this case, in fact, it becomes necessary to correct the phase jitter of the baseband signal; to obtain this, as known, it is necessary to supply to the phase corrector not only the bandband signal but also a linear conversion thereof.
To solve these problems, two theoretical solutions are known in the technical literature; they differ from one another on the limitations imposed upon the sampling rate as well as in regard to the sampling mode utilized.
According to the first solution, either the ratio f.sub.s /f.sub.c between sampling rate f.sub.s and the minimum transmitted frequency f.sub.c (in the case of f.sub.s .gtoreq. f.sub.c) or the inverted ratio f.sub.c /f.sub.s (if f.sub.s .ltoreq. f.sub.c) must be an integer, in particular 1. A solution of this kind could directly provide the baseband samples of the received signal, as it is easy to demonstrate, and could be implemented by a simple sampler; but such a procedure presents serious disadvantages.
Above all, aliasing can be avoided only if the modulated signal has a very narrow bandwidth, and by using an ideal filter in order to separate repetitive spectra; as, in the practice, these two conditions and especially the latter one cannot be satisfied, a certain residual superposition or aliasing effect will always be present in the demodulated signal and will give rise to distortions of the signal itself.
Moreover, it is evident that, as the same device (i.e. the sampler) must perform the demodulating operations and ensure the bit timing, in the event that the transmission channel varies with time, demodulating and timing operations ought to be made adaptive; this can easily produce interactions degrading the operation of the system.
Furthermore, in the event that there is a frequency offset of the band of the transmitted signal, the minimum frequency f.sub.c is no longer exactly known, and so the sampling rate f.sub.s no longer satisfies the condition necessary for the direct demodulation of a baseband signal.
The latter disadvantage, at least in the case where the sampling rate is higher than the minimum transmitted frequency, could be made less significant by choosing a high value for the ratio between the two aforementioned frequencies. In that instance, however, further problems arise from the necessity of using high-velocity equipment difficult to realize, or are associated with the identification of the samples carrying useful information.
In the particular case where the minimum transmitted frequency is an integral multiple of the bandwidth of the modulated signal (which is convenient from the constructional point of view when the transmitter is of digital type), the sampling rate is subject to further restrictions which very often are incompatible with the structural requirements of the transmission system depending on the particular requirements of use.
According to a second way of maintaining the digital structure of the demodulator, the ratio between the sampling rate and the minimum transmitted frequency must be different from an integer.
This procedure always allows to keep the information contents associated with the transmitted signal and make it possible, thanks to the more flexible relationship between the two frequencies, to overcome frequency-offset and aliasing effects with greater flexibility.
More particularly, by a suitable choice of the values of ratio f.sub.s /f.sub.c, there is no need for using an ideal filter for separating adjacent spectra.
This system, however, yields only partly demodulated signal samples which, besides, appear at the downstream components of the receiver with a timing rate different from the bit rate.
If the only partial demodulation can be completed through a suitable phase corrector, always necessary in baseband to obtain a perfectly coherent demodulation, the presence of samples arriving one after the other at a timing rate different from the bit rate makes it necessary to insert, past the sampler, devices able to emit samples recurring again at the bit rate; but such devices have not yet been satisfactorily realized by the conventional techniques.
The above-mentioned difficulties, relative to the practical realization of the demodulation by in-band sampling, have caused the art to tend toward the sampling of already analogically demodulated signals.
This solution presents the disadvantage of leading to receivers in which both digital and analog components are present. This causes problems of integration between the two kinds of components which can be solved only by using suitable interfaces, making the whole device complex and thus not very reliable as well as limitedly flexible on its performance.
Moreover, a hybrid (i.e. analog-digital) system is unable to provide the linear conversion of the baseband signal necessary for the operation of the phase corrector. This makes it necessary to equip the receiving set with an auxiliary device, able to carry out such linear conversion, which presents serious design difficulties.